In our recent experiments, we have shown that mesenteric lymph node cells from DNP-ovalbumin-primed rats responded in vitro to homologous antigen to develop IgE, IgG and IgM-forming cells. In this system, the addition of T cells from rats infected with Nippostrongylus brasiliensis (Nb) specifically potentiated IgE to DNP-OA without affecting IgG response. Subsequent experiments suggested that T cells from Nb-infected rats form soluble factor(s) which potentiates the IgE response. Experiments will be carried out to study the relationship between the IgE-potentiating factor and T cell-replacing factor which are released by the stimulation of carrier-primed cells with homologous carrier-primed cells with homologous carrier. Target cells of the IgE-potentiating factor will be studied as well. In a separate study, we shall investigate ontogenetic development of IgE-bearing lymphocytes in the mouse, using immunofluorescence.